In Fourier transform type spectroscopic devices, interferometers are used to detect interference waves of measurement light in a non-dispersive method, and interference waves are Fourier-transformed by a computer to obtain spectrum data of measurement light. Due to formation of interference waves of measurement light, each wavenumber components of measurement light based on intensity signals of interference waves composed of total wavenumber component of measurement light can be calculated by Fourier transform. Fourier transform spectroscopy is suitable for high speed measurement, and is the mainstream in infrared spectrophotometers. It is widely used as Fourier transform type infrared spectrophotometers (FTIR) (refer to Patent Literature 1).
The interferometer used in this spectroscopic device is generally a Michelson interferometer, and comprises a semi-transparent mirror and two reflection mirrors (fixed mirror and movable mirror). The movable mirror makes the optical path difference of the interferometer variable, and the position of the movable mirror and the optical path difference are in a one-to-one relation. The interferometer generates interference waves of measurement light that correspond to the optical path difference. By detecting intensity of this interference wave, an interferogram (interference curve) having the optical path difference in a horizontal axis and the intensity signal in a vertical axis can be obtained. The computer calculates the spectrum data by Fourier-transforming the interferogram data.
The timing of detecting the interference wave is generally determined by referring to the position of the movable mirror by a laser light. It is referred to as a position reference laser herein. The position reference laser irradiates the laser light to the movable mirror, and the interferometer forms a laser interference wave based on its reflection light. Usually, a laser detector detects the interference wave of the measurement light when the intensity signal of the laser interference wave becomes zero. That is, the detection timing depends on a wavelength λ of the position reference laser.
On the other hand, in an FTIR of Patent Literature 1, a ΔΣ type AD converter AD-converts the interferogram signal to improve S/N. This ΔΣ type AD converter uses a basic clock that is asynchronous with moving speed of the movable mirror to AD-convert the interferogram signal from the infrared detector and the laser interference wave signal from the laser detector simultaneously. Then, the computer interpolates the interferogram signal that is AD-converted so that the angle variation of the laser interference signal is constant, and thus obtains the interferogram that is synchronized with the movement of the movable mirror.